The invention relates to an automatic lathe which is designed as a so-called counterspindle machine, i.e. an automatic lathe comprising a machine base and at least two workpiece spindles which are each provided with a workpiece clamping means and are adapted to be positioned with coaxial spindle axes and workpiece clamping means facing one another. In counterspindle machines of this type, the workpiece spindles are not usually movable at right angles to the spindle axis, i.e. neither of the two workpiece spindles has a so-called X-axis or Y-axis; the invention can, however, also be used for those lathes, in which one or both of the workpiece spindles is or are displaceable in the direction of the X-axis and/or the Y-axis.
Counterspindle machines of the type in question are known, for example, from DE-PS 35 14 069, DE-PS 30 35 451 and EP-B-0 310 805. They enable a workpiece to be machined completely in two settings. In this respect, the workpiece is held first of all in the clamping means of the one workpiece spindle and the workpiece portion protruding from this clamping means is machined. Thereupon, the workpiece is taken over by the other workpiece spindle and the workpiece area which is not machinable in the first step is now machined. In order, above all, to transfer the workpiece from the one workpiece spindle to the other at a correct angle of rotation, the workpiece must be clamped in the clamping means of the second workpiece spindle before the clamping means of the first workpiece spindle is opened.
Bar stock, i.e. bar-like raw material, is often processed on such counterspindle machines. For this purpose, at least the workpiece spindle performing the first machining steps is designed as a hollow spindle, through which stock bar is pushed from behind, i.e. from the side of the first workpiece spindle facing away from the counterspindle, through the first workpiece spindle as far as the operating space of the automatic lathe located between the two workpiece spindles and then clamped in the clamping means of the first workpiece spindle. Following machining of the forward region of the stock bar which protrudes out of the first workpiece spindle and later forms the workpiece, the counterspindle is pushed with its clamping means over the machined, forward region of the bar stock and this is clamped in the counterspindle. Subsequently, the bar region forming the later workpiece is severed from the stock bar by means of a so-called parting tool, both workpiece spindles being driven at the same rotational speed with their clamping means closed. The region on the rear side of the workpiece held in the counterspindle can then be machined.
When machining bar stock, it is often necessary to feed the stock bar in the direction towards the counterspindle with its help. In the case of very long workpieces, in particular, it is not possible to advance the stock bar through the first workpiece spindle against a stop located outside the counterspindle, namely when the stock bar has to be pushed forward to such an extent that its front end comes to rest outside the operating space of the automatic lathe and inside the counterspindle or its clamping means. In these cases, the front end of the stock bar is clamped in the counterspindle before it is advanced, whereupon the clamping means of the first workpiece spindle is opened and the counterspindle moved back by a distance which corresponds to the desired bar feed.
In all these cases, it becomes disadvantageously apparent in conventional counterspindle automatic lathes that the workpiece clamping means, such as so-called tension or pressure collets, which are customarily used are constructed such that, during a clamping action, their clamping zones which engage on a part to be clamped in them undergo a movement which has a movement component parallel to the spindle axis. During the transfer of a workpiece at the correct angle of rotation, the clamping means of the counterspindle must be closed before the clamping means of the first workpiece spindle is opened; the same applies to the cutting off or severing of the later workpiece from the stock bar. Also, when the stock bar is fed with the aid of the counterspindle, the clamping means of the first workpiece spindle must be closed before the clamping means of the counterspindle is opened. In all these cases, the later workpiece, while one of the two workpiece clamping means is being closed, is displaced in the direction of the spindle axis relative to the clamping zones of at least one of the two clamping means and this leads to undesired markings on the workpieces produced on the automatic lathe as well as to inaccuracies in the length of the workpieces measured in the direction of the workpiece spindle axis. When cutting off the later workpiece from a stock bar by means of a parting tool, this characteristic of the workpiece clamping means normally used can also lead to the parting tool becoming jammed or deflected in an undesired manner in the direction of the spindle axis since, in order to achieve machining times per piece which are as short as possible, the clamping procedure is not completely terminated when the parting tool begins operation.